This invention relates to a method of producing coke for metallurgy, and more particularly a method of producing high-strength coke for metallurgy capable of using in a large size blast furnace by blending a great amount of brand coal near to the quality of a coal blend for the charge in a coke oven to form the coal blend consisting of only a few brands of coals without blending many brands of coals.
When making molten iron in a blast furnace, it is first necessary that iron ores and coke are alternately charged into the blast furnace and filled therein in the form of layers. The iron ore and coke are heated by hot air of high temperature blown through a tuyere and at the same time the ore is reduced to iron by CO gas generated through combustion of coke.
In order to stably conduct the operation of such a blast furnace, it is required to ensure air permeation and liquid permeation in the furnace, and hence it is inevitable to use coke having excellent properties such as strength, particle size, strength after reaction and the like. Among them, the strength (drum strength) is considered to be an important property.
In the production of such coke for the blast furnace, it is required to carbonize the coal blend (charging coal) for charging into a coke oven having a constant coking property and coalification degree. For this purpose, a raw coal having a good quality (which is mainly called by a production area, and is called as a brand) is necessary. Recently, such brands of coals (hereinafter referred to as a xe2x80x9craw coalxe2x80x9d) is difficult to obtain in a great amount. Therefore, there has been used a so-called xe2x80x9ccoal blendxe2x80x9d obtained by blending many kinds of the raw coals having different properties in accordance with the production country and production area (usually 10xcx9c20 brand coals).
In such a coal blend, it is common to blend coal of one brand in an amount of not more than 20 wt % at most. This blending thought lies in that the raw coals are blended so that a quality of coke obtained by carbonization of the coal blend in a coke oven is made to be not less than a certain level. For example, it is enough to balancedly blend fibrous components forming a skeleton of coke (which is evaluated by the coalification degree of coal using volatile component, C wt %, vitrinite reflectance and the like as an indication) with coking component forming an aggregate through coking of coal particles (there are fluidity of coal, expansion degree, tackiness index and the like as an indication). That is, the strength of coke after carbonization is guessed obtained by calculating the quality as the coal blend based on coalification degree and coking property of each brand of raw coals.
At the present, 10xcx9c20 brands of raw coal are usually blended as a coal (coal blend) charged into the coke oven used for the production of coke for a blast furnace. According to this method, the influence of the properties of the raw coal per one brand upon the qualify of the coke as a final product becomes small. Therefore, even in the case of coal unsuitable for the production of coke for blast furnaces, it may be blended only in a small amount, and serves to stabilize the quality of coke as a merit.
As to the raw coals blended for the production of coke for blast furnace, however, it is presently used to select only coals having relatively good quality as compared with coal used for the production of general-purpose coke. Therefore, the iron-making technicians are always troublesome in the saving of good quality coals as it is.
Among the raw coals being cheap and available in a greater amount, for instance, there is medium coking coal having a high content of inert component indicating a mean reflectance of 0.9xcx9c1.1 and a maximum fluidity of not more than 3.0. And also, such raw coals have substantially the same quality property as in the above usual coal blend. According to the inventors"" study, however, when a greater amount of this raw coal is blended and carbonized, the desired coke strength can not actually be obtained though the quality is similar to that of the coal blend, and hence it is difficult to use it in a greater amount.
On the other hand, according to the conventional method of blending many kinds of raw coals having a certain quality, e.g. about 20 brands of coals must be always stocked in a coal yard. This causes problems in the yard site such as the cost for unloading and quarrying becomes expensive and the like.
In the conventional technique, it is required to adjust and blend many brands of raw coals as a coal blend to be charged into the coke oven as mentioned above. However, the raw coal to be blended is difficult to get in accordance with the brand thereof, or even if such raw coals are obtained, there is a problem in the maintenance of the raw coals in the stock yard.
Under the above circumstances, it is, therefore, an object of the invention to provide a method of advantageously producing coke for metallurgy having an excellent quality such as strength and the like as compared with the conventional method, particularly high-strength coke capable of use in a large-size blast furnace by blending a greater amount of a brand of a raw coal being cheap and easily available with several brands of raw coals.
The inventors have made various studies with respect to the kinds of raw coals and the blending thereof in order to achieve the above object and found that there is a combination suitability or affinity in a combination of so-called particular brands of raw coals because the coke strength is largely shifted from that estimated from a weighted mean value of each raw coal in accordance with the method of combining raw coals of different production countries (each brand coal). That is, it has been confirmed that the strength required as a coke for metallurgy is obtained by utilizing the affinity of particular brands of raw coals with the other brand of raw coals even if the raw coal is restricted to a few brands and these brands are blended, and as a result the invention has been accomplished.
That is, the invention lies in a method of producing coke for metallurgy by blending plural brands of raw coals to form a coal blend and carbonizing it in a coke oven, characterized in that a coal blend containing not less than 60 wt % of medium coking coal having a content of inert component of not less than 30%, a middle coalificiation degree and a low fluidity is used as a coal charged into the coke oven.
In the invention, the above medium coking coal of middle coalification degree and low fluidity is favorable to have a equilibrium moisture content of not less than 3.5%.
In the invention, it is favorable that the coal blend consists of 60xcx9c95 wt % of the medium coking coal having the middle coalification degree and low fluidity and 5xcx9c40 wt % of hard coking coal and/or medium coking coal having a high coalification degree and/or a middle-high fluidity.
In the invention, it is favorable that one or more raw coals having an mean reflectance (R0) as the coalification degree of 0.9xcx9c1.1 and a maximum fluidity (MF) as a coking property of not more than 3.0 are used as the medium coking coal having the middle coalification degree and low fluidity.
In the invention, it is favorable that either one or more of high coalification coking coal having an mean reflectance (R0) as the coalification degree of not less than 1.3 and middle-high fluidity coking coal having a maximum fluidity (MF) of not less than 3.0 are used as the hard coking coal and/or medium coking coal having the high coalification degree and/or middle-high fluidity.
In the invention, the product coke is favorable to indicate of a tumbler strength (TIE) of not less than 83%.
According to the method of the invention having the above construction, raw coals being cheap and available in a great amount can be blended in a greater amount, so that it is possible to stably ensure coke for a large-size blast furnace having an excellent quality indicated by TIE of not less than 83%, preferably not less than 84% even when the coal blend is formed by blending raw coals of brands smaller than the conventional brand number.